Lengthen an edge infinitely (a.k.a. vanishing point)
Since your screen resolution is far from infinite, moving points far away will quickly reach the pixel size of your display. Starting with a 2m default cube, extruding an edge back 5000m should be plenty enough to simulate infinity and have them join at a subpixel level. Before doing this, don't forget to increase the Clip End limit of your viewport of camera.

Here is a simple way to do it in the view port: